Make GetReverseUnicodePosition less horribly awful
Fix selection preserving in SubsTextEditCtrl::SetTextTo

Calling GetReverseUnicodePosition on the selection positioning after
calling SetText resulted in the conversions between UTF-8 indices and
wxString indices just being a very slow no-op.
Make DialogTranslation::UpdateDisplay a bit less slow
Support reading keyframes files from ffmpeg/avconv
Fix test compilation